What's the judgment on a Muslim who calls other Muslims because of their doings Kaffara?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Brother inquirer!
Know that accusing someone of unbelief is a serious matter, especially if one recognizes that a disbeliever will be in the Hell-fire on the Day of Judgment. Also, the disbeliever will lose (i.e. be divorced from him) his wife and his portion of inheritance. Then, he is to be killed due to being a disbeliever. We ask Allah to save us all.
Thus, disbelief is a special serious ruling. No one should be accused of unbelief unless he commits one of the matters that make him disbeliever without any form of doubt or interpretation. Among matters that make one a disbeliever are:
(1) Mocking at Allah's Messenger and (2) insulting the Qur'an. But, here are two matters that prevent us from accusing him of unbelief. They are:
1. Mocking at the Messenger or insulting the Qur'an under compulsion.
2. Being ignorant, since he is a new Muslim or growing up among non-Muslims.
Here, we state that we have to take the above-mentioned points into our consideration before accusing someone of disbelief and we have to teach him. It is reported by al-Bukhari and Muslim from Ibn 'Umar (Radiya Allahu Anhu) that Allah's Messenger (Sallallahu Alaihi wa Sallam) said: "When a man calls his brother as 'O Kafir' (disbeliever) the epithet must rebound upon one of them. If the person thus addressed is such (then he is a disbeliever) otherwise it rebounds upon him who has said so" .
Another Hadith from Abu Zar (Radiya Allahu Anhu) reads: "If a person calls another person Kafir or an enemy of Allah and the latter is not so, then, the epithet will rebound upon the former" [Reported by Imams Bukhari and Muslim ].
Allah knows best.